In recent years, a computer to plate system (CTP), in which image data can be directly recorded in a printing plate material, has been widely used accompanied with the digitization of printing data. As a printing plate material usable for CTP, there are a printing plate material comprising an aluminum support such as a conventional PS plate, and a flexible printing plate material comprising a flexible resin film sheet and provided thereon, various functional layers. Recently, in the commercial printing industries, there is a tendency that many kinds of prints are printed in a small amount, and an inexpensive printing plate material with high quality has been required in the market. As a conventional flexible printing plate material, there are a silver salt diffusion transfer type printing plate material as disclosed in Japanese Patent O.P.I. Publication No. 5-66564, in which a silver salt diffusion transfer type light sensitive layer is provided on a flexible sheet; an ablation type printing plate material as disclosed in Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773 in which a hydrophilic layer and a lipophilic layer, one of which is the outermost layer, are provided on a flexible sheet where the outermost layer is ablated by laser exposure to prepare a printing plate; and a heat melt type printing plate material as disclosed in Japanese Patent O.P.I. Publication No. 2001-96710 in which a hydrophilic layer and a heat melt image formation layer are provided on a flexible sheet where a hydrophilic layer or a heat melt image formation layer is imagewise heated by laser exposure to heat-fix the image formation layer onto the hydrophilic layer.
The silver salt diffusion transfer type printing plate material requires a wet development step and a drying step after exposure, which does not give sufficient dimensional accuracy during the image formation step, and is not suitable to obtain printed matter with high image quality.
The ablation type printing plate material does not require a wet development step, but image formation due to ablation is likely to fluctuate in dot shape. Further, there is problem in which the interior of the exposing apparatus or the printing plate surface is contaminated by scattered matters caused by ablation of the layer.
A process, comprising a step of forming on a hydrophilic layer a heat-melted image, heated by conversion from laser light, is suitable to obtain high precision images. Among types of this process, there is a so-called on-press development process in which when a printing plate material after image writing is mounted on an off-set press, and a dampening solution is supplied to the printing plate material during printing, only the image formation layer at non-image portions is swollen or dissolved by the a dampening solution, and transferred to a printing paper (paper waste), whereby the image formation layer at non-image portions is removed. This process does not require a special development after exposure, resulting in excellent stability of printing quality and excellently meeting environmental concern.
Generally, a printing plate material having a plastic film sheet as a support is placed in the roll form in an output apparatus and automatically cut to a given size there. A printing plate material for CTP having a plastic film sheet as a support may have a back coat layer for controlling electroconductivity, friction or surface shape on the surface of the support opposite the image formation layer in order to fix easily the printing plate material on an exposure drum or a plate cylinder.
When a printing plate material is wound around a core in roll form, the back coat layer and the image formation layer contact each other, and therefore, it is necessary that both layers be unaffected by each other. A method has been proposed which a matting agent is incorporated in a back coat layer of a printing plate material to control its coefficient of friction or surface shape of the back coat layer (see for example, Japanese Patent O.P.I. Publication No. 11-91256).
The technique disclosed in the Patent document above is a printing plate material in which when the printing plate material is wound around a core in the roll form, and stored for a long time, a spotted pressure due to protrusions derived from the matting agent is applied to the image formation layer, resulting in lowering of printing image quality such as stain occurrence at non-image portions. It has been found that particularly in a printing plate material of the on-press development type comprising a plastic sheet support, a functional layer comprised of a hydrophilic layer and a thermosensitive image formation layer provided on one side of the support, and a back coat layer provided on the other side of the support, which is put on the market in the roll form, there occur phenomenon that development speed partially decreases and ink transfer to image portions is non-uniform. Further, it has been found that particularly when ink containing no petroleum volatile solvent (for example, soybean oil ink), which has been widely used in view of environmental protection, is employed, ink transfer to image portions is more non-uniform.